An example of methods for reducing errors at the time of rough seeking in conventional optical disc apparatuses is a method in which the number of tracks to be sought is calculated from a current address and a target address, and when the number of tracks to be sought is equal to or more than a predetermined number that needs a rough seek, an optical head is moved only by a distance obtained from the number of tracks to be sought and a track pitch, using a moving unit such as a traverse motor (a stepping motor) or the like. At this time, in order to reduce errors in the moving distance caused by variance in the track pitch, a correction is provided when the number of tracks to be sought is converted to the moving distance so that the convergent point of the optical beam will reach closer to the target position in the following rough seek, taking into account the difference between the number of tracks at the position that the convergent point of the optical beam reached and the number of tracks at the target position in the last rough seek (see JP2002-329335A). The rough seek means to move the convergent point of the optical beam to a target position with comparatively low precision based on the number of tracks to be sought calculated from the current address and the target address.
In the above-mentioned conventional optical disc apparatuses, however, in the case where an optical head with another lens disposed at a position shifted from a traverse axis in a tangential direction (in a direction of a tangent to an optical disc) is used, like an optical head having dual lenses (an optical head with a dual-lens structure), for example, when a rough seek (a traverse movement) is performed only based on the moving distance obtained from the track pitch and the number of tracks to be sought calculated from the addresses, the actual lens position will be moved by the number of tracks that is different from the calculated number of tracks. Although the exact definition of the traverse axis is as described later, it briefly can be described with some concrete terms that the traverse axis here means a virtual axis that is parallel to a direction in which the optical head is moved by the traverse motor or the like and that passes above the center of the optical disc (corresponding to the center of the disc motor on which the optical disc is mounted). The lens here means a converging lens that converges an optical beam onto a predetermined position on the optical disc. As long as the lens provided in the optical head moves along the traverse axis (above the traverse axis), the lens will move along a radius of the optical disc. Therefore, once the current address and the target address are known, a radial distance and the number of tracks from the current address to the target address can be calculated assuming that the track pitch is fixed. However, when the course of movement of the lens is shifted from the traverse axis, the above-mentioned calculation does not hold and an error occurs with respect to the actual number of tracks. Drawbacks have been that such an error is too difficult to be corrected by a level of correction designed for absorbing a track pitch variance, and as a result, another seek is needed and a fine seek to the target address takes time, increasing the entire seek time. The fine seek means to move the convergent point of the optical beam to a target position by tracking control.